If simply blocking use of a web browser is sufficient, you could set the permissions on iexplore.exe and any other browsers you have to deny access to the Guest account. This would effectively prevent them from using a web browser (assuming you have no others installed). You could also deny access to other internet-based applications.

Just keep in mind that this will not guarantee the user can't access the internet, only inhibit general users. Even a modestly skilled user could bring his thumb drive with his own browser and run it from the guest account.